Help with Timing Please...

[74bigblock]

*Disclaimer... I know nada about timing, except how to read and set it. So my post is really a timing 101 issue... please read on:

According to the manual the timing for the 1974 LS4 should be 10 deg before TDC. I know enough to set the timing at this mark, or any mark, but that's about the extent of my knowledge. Correct me if I am wrong; advancing the timing can increase the performance of the motor (power wise, not fuel efficiency wise; and let's face it... it's a big block... like I care if I get 11 MPG or 10!).

My questions:

-Would advancing the timing hurt the motor?
-What are the reasons (besides the obvious) that I should advance the timing (should I account for any engine or ignition modifications)?
-If I advance the timing... would that be on the 12 deg before TDC or the other way to 8 deg before TDC?
-Is there such a thing as too much advance/too little advance (if so, what are the symptoms)?

Please chime in with your thoughts!

Thanks as always!
Dave

 

[Vettefan87]

-Would advancing the timing hurt the motor?

Depends how much advance you have before TDC.

-What are the reasons (besides the obvious) that I should advance the timing (should I account for any engine or ignition modifications)?

Yeah, you will have to play around with it a little since your car is modded, you will have to find out what works best with your setup.

-If I advance the timing... would that be on the 12 deg before TDC or the other way to 8 deg before TDC?

I am not sure what you mean by this question, what do you mean other side, do you mean the side that retards the timing?

-Is there such a thing as too much advance/too little advance (if so, what are the symptoms)?

Yes, there is. If you have to much advance your car will spark knock, timing is tricky, you just have to play around with it, is what I have learned.


These are just what I know personally from working on my car, and reading on the forums. Someone will be in here with more info then me

 

[74bigblock]

Depends how much advance you have before TDC.

-Right now she is at stock 10 deg Before TDC. Runs fine.

I am not sure what you mean by this question, what do you mean other side, do you mean the side that retards the timing?.

-If I advance the timing, do I want to move to 12 deg before TDC side, or to 8 deg before TDC side?

Yanks! -D

 

[Vettefan87]

Sorry I read that wrong, I am very tired and sore, pretty much two straight days of trying to get headers on my car has worn me out, but I would definetly not take it any lower then what stock calls for, unless you are running lower compression, then you might want to, I think. If you are running open headers as I know since I just installed headers. It is LOUD, and you might not be able to hear spark knock either. You could try and play with it, I know my car would spark knock like crazy with anything over stock which is 6 degrees before TDC. You might have to run like 100 octane that is unleaded, to deal with the added timing if you really want more timing.

 

[LT4man]

One thing that a lot of people do not realize: you can time an engine with a vacuum gauge. This works well with an engine that has a lot of overlap in the camshaft. This engine would not produce a lot of vacuum, but still could be timed using a vacuum gauge. :confused

The stock setting for ignition timing is the right place to start. Too much advance will cause pinging (sometimes called detonation, but more correctly called pre-ignition!). A more common term is knocking. All of the before-mentioned things are BAD things and should be avoided at all costs. ad

Save The Wave!

 

[LT4man]

Moving to 12 would be MORE advance. Moving to 8 would be LESS advance.

So, if you want to advance the timing, you would move towards 12.

Hope that helps!

 

[LT4man]

After tightening the distributor hold down, I would definitely put a piece of tape on it!

I am so sorry. I just could not stop myself!

 

[Vettefan87]

This is something that is easily done with two people. One moving the distributor, and one watching the light. Get it to 11, or 12 wherever you want it. Tighten down the bolt, then cheack it again and see if it moved, then you are good to go.

 

[74bigblock]

After tightening the distributor hold down, I would definitely put a piece of tape on it!

I am so sorry. I just could not stop myself!

LOL.... I love tape.

 

[LT4man]

I know you do. I do too. What I am trying to figure out is: where did you get your love of tape from? I understand if it is too personal to share!

 

[74bigblock]

The digression from timing to tape...

When I was a kid I used to use tape to fix everything. I would make sculptures out of soda straws and tape. When I was in 5th grade I made this cool art project with an old bottle. I covered the glass with little pieces of masking tape, then rubbed shoe polish all over it. It made a neat antuiqe look.


So now I just say... if it's broke... TAPE IT!

 

[Jack]

Ignition timing is usually referenced by how many degrees from TDC (top dead center aka 0/zero degrees). So if factory setting calls out 10 degrees BTDC (before top dead center) that means it fires 10 degrees before (advanced) TDC (zero) ... so, when standing at front of car looking down at timing tab & balancer and starter motor is to your left ... the timing mark should align with the 10 degree tick mark on timing tab. BTW ... timing tab tick marks are usually 2 degrees apart. So, find the zero/tdc tick and then move to left (advance) 5 ticks ... the balancer's timing mark should align there for 10* advanced (BTDC) timing. If you have a dial up timing light, set it on 10* advanced and timing mark should align with zero/tdc tick. For 12* advanced look for alignment at 6 ticks to left of zero/tdc.
JACK:gap

edit
DAVE: To further fulfill your penchant for "tape" ... you can get a "timing tape" that adheres to your harmonic balancer/damper ... it'll have degrees marked on it up through about 60* advanced/btdc ... set your timing and sate your fetish in one fell swoop!

 

[Bob Chadwick]

Dave,

There is a lot of information on this for small blocks but I don't think I have seen anything on big blocks. The principle would be the same, you just need to determine the optimum advance curve for a big block.

On the SBC, you want to obtain 36 degrees of mechanical advance at or just under 3,000 rpm. People appear to be getting this in two different ways; by either advancing the inital timing to get there or by messing with the distributor internals to change the springs and curve.

I did mine with the latter method, but I'm not 100% sure how you do it with a stock distributor because I needed a new distributor and replaced it with an MSD unit. All I had to do was put certain springs in that controlled how fast the advance came in and a bushing to control how far it went.

If you do it the first way by advancing the initial timing make sure the total mechanical advance does not exceed the recomended amount. I think that you could get the recommended advance in by the recommended rpm but have additional advance come in later due to spring or stop configuration.

I think what you need to determine first is what the optimal advance is and at what point it should be fully advanced. Mayve Hib or JohnZ can help you out or try Lars at the CF. Very nice guy and very helpful.

You don't say but I assume you are disconnecting and plugging the vacuum line to the vacuum advance unit on the distributor.

 

[JohnZ]

One thing that a lot of people do not realize: you can time an engine with a vacuum gauge. This works well with an engine that has a lot of overlap in the camshaft. This engine would not produce a lot of vacuum, but still could be timed using a vacuum gauge. :confused

The stock setting for ignition timing is the right place to start. Too much advance will cause pinging (sometimes called detonation, but more correctly called pre-ignition!). A more common term is knocking. All of the before-mentioned things are BAD things and should be avoided at all costs. ad

Save The Wave!

Timing an engine with a vacuum gauge isn't a good technique, and is hopelessly inaccurate, especially with a high-overlap camshaft that produces low and unsteady manifold vacuum at idle. The only way to time an engine properly is with a timing light.

Detonation and pre-ignition are two totally different phenomena, although they produce the same results; pre-ignition occurs before the spark plug fires, generally a result of "hot spots" that ignite the incoming charge, and detonation occurs after the plug has fired; the "knocking" sound you hear with detonation is the collision of the primary and secondary flame fronts - the normal one beginning at the plug, and the self-induced secondary one beginning elsewhere in the combustion chamber.

 

[brusso]

I'm glad that no one has started the ported vs manifold vaccum discussion.:eyerole

 

[JohnZ]

Dave, I wrote a 2-part series on ignition timing in the October/November (combined issue) and December 2003 issues of "Corvette Enthusiast" magazine that explains all about it and how to work with it; the subject is too complex to put in a forum post, but I wrote the articles in terms that guys of any skill or experience level can deal with successfully at home with normal tools.

I could e-mail you the text of the articles, but they're a LOT more meaningful with the photos; you can order the back issues by calling (800) 572-6885. Give that a shot and I'd be glad to answer any questions you have after reading the articles.

 

[Ken Anderson]

If it's a big block, then all you need is a dial back timing light and set it at 38 degrees and forget it. Plug your vac ad, get light springs and have it all in by 2800-3000 RPM.
This will give you maximum torque and is what Chevy recommends for racing.

 

[JohnZ]

If it's a big block, then all you need is a dial back timing light and set it at 38 degrees and forget it. Plug your vac ad, get light springs and have it all in by 2800-3000 RPM.
This will give you maximum torque and is what Chevy recommends for racing.

True, but that assumes the centrifugal curve is somewhere near correct (quick and easy enough to determine with a dial-back light), but I'd want to verify the curve first; if the centrifugal curve is short (in degrees), he could end up over-advanced at idle/low speed. You wouldn't believe what I've found in some distributors, either through neglect or lack of knowledge.

 

[Ken Anderson]

True, but that assumes the centrifugal curve is somewhere near correct (quick and easy enough to determine with a dial-back light), but I'd want to verify the curve first; if the centrifugal curve is short (in degrees), he could end up over-advanced at idle/low speed. You wouldn't believe what I've found in some distributors, either through neglect or lack of knowledge.

I believe your last statement completely and that pretty much sums it up.

 

[lucky76]

After tightening the distributor hold down, I would definitely put a piece of tape on it!


I am so sorry. I just could not stop myself!

What's the tape for?:blue: